With a title like Heisenberg, it bears noting that Simon Stephens's drama, which Burning Coal Theatre Company opened last weekend a month after it closed on Broadway, isn't some scientific historical thriller like Michael Frayn's Copenhagen. For what it's worth—and it's actually worth quite a bit—the two-hander also lacks the dazzling intellectual fireworks of Alan Lightman's speculative novella, Einstein's Dreams (in which director Emily Ranii starred when Burning Coal produced its original stage adaptation in 1998).

But while Stephens keeps the mental calisthenics about quantum mechanics more subdued, the quiet poetics he finds in Heisenberg's famed uncertainty principle has room to breathe. So do the characters: Alex, a diffident, seventy-five-year-old London butcher, and Georgie, an enigmatic woman in her early forties. The two strangers meet when Georgie interrupts Alex's people-watching at a train station with a kiss on the back of his neck.

The rushed recital of apologies, casual insults, and rapidly unraveling explanations that ensues reveals Georgie as a woman whose backstory (as an assassin—no, a waitress) has a few too many alternative facts to be truly trusted. As she pursues a singularly awkward introductory conversation—and continues to pursue Alex afterward—we see her tricky reversals of opinion. When Alex asks if she's spying on him, she truthfully answers, "I am. I'm not really. I am, kind of."

As the relationship develops, Stephens presents Georgie as an entity who frequently occupies two contradictory states of mind, if not identity. Call her Schrödinger's girlfriend, for lack of a better term.

At its heart, Heisenberg's uncertainty principle proves there is a limit to the knowledge we humans can attain about our surroundings and our universe, and that this border is determined not by the sensitivity of our instruments but by the nature of existence itself. In Georgie's badly damaged relationship with her son, in Alex's informed meditations on a Bach sonata and his utter reticence in the realm of feelings, and in the ever-changing intersection of these two improbable characters, the subtle metaphor unwinds: the more precisely you can locate a particle—or a person—the less you can actually know about his or her trajectory or momentum.

Under Ranii's probing direction, Sarah Hankins fully occupies the prismatic quantum states of Georgie, and Tom McCleister gradually brings Alex's hidden depths to the surface. Accompanied by David Ranii's fitting, sometimes discordant audio mix, the pair explores the interiors of several relationships—to the point beyond which only mystery lies. Highly recommended.